Remove the ends of the asparagus if they are stringy. You can do this by bending about an inch or two from the bottom. They stringy end should snap off.
Finely slice the mushrooms and defrost the peas in hot water.
Put the gram flour and HALF the plant milk in a bowl with salt and stock powder. Mix with a whisk until the lumps have disappeared. Add the remaining milk.
Finely chop the fresh tarragon, add it to the bowl with the vegetables and mix in well.
Heat a wide frying pan on a medium to high heat. Add the oil, once it is very hot add the contents of the bowl, spending it out across the pan.
Cook for around 15 minutes until the underside is golden brown. Add cheese (if you are using it) then put the frittata under a hot grill for another 10 minutes until the cheese is melted and the tortilla is cooked through. Serve immediately.